I may be
wrong, but you may also have to open up the default sound recorder to
hear music on your speakers from the line in... I seem to remember
having to do that, but sound has always been a little screwy on my
machine. A (likely) more complicated way to listen to music on the
line in is to install Audacity and JACK. They are both in the
repositories. The only benefit here is that you get some extra sound
tweaking options to make it sound better, especially if you have some
great speakers on your box. The down side is (to me) JACK is
ridiculously complicated, and I can only seem to get it to do what I
want by trial and error. When I go to record something these days, I
just start up Audacity, and pray that whatever setting I had on JACK
server previously are still going to be intact and working when I
start it. I don't know why it seems like such a crap shoot...